Flattened Tube Finned Heat Exchanger And Fabrication Method

ABSTRACT

A multiple tube bank heat exchanger includes a first tube bank including at least a first and a second flattened tube segments extending longitudinally in spaced parallel relationship and a second tube bank including at least a first and a second flattened tube segments extending longitudinally in spaced parallel relationship. The second tube bank is disposed behind the first tube bank with a leading edge of the second tube bank spaced from a trailing edge of the first tube bank. A continuous folded fin extends between the first and second flattened tube segments of both of said first tube bank and said second tube bank.

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to and this application claims priority from and thebenefit of U.S. Provisional Application Ser. No. 61/548,864,filed Oct.19, 2011, and entitled FLATTENED TUBE FINNED HEAT EXCHANGER ANDFABRICATION METHOD, which application is incorporated herein in itsentirety by reference.

FIELD OF THE INVENTION

This invention relates generally to heat exchangers and, moreparticularly, to flattened tube and fin heat exchangers.

BACKGROUND OF THE INVENTION

Heat exchangers have long been used as evaporators and condensers inheating, ventilating, air conditioning and refrigeration (HVACR)applications. Historically, these heat exchangers have been round tubeand plate fin (RTPF) heat exchangers. However, all aluminum flattenedtube and fin heat exchangers are finding increasingly wider use inindustry, including the HVACR industry, due to their compactness,thermal-hydraulic performance, structural rigidity, lower weight andreduced refrigerant charge, in comparison to conventional RTPF heatexchangers.

A typical flattened tube and fin heat exchanger includes a firstmanifold, a second manifold, and a single tube bank formed of aplurality of longitudinally extending flattened heat exchange tubesdisposed in spaced parallel relationship and extending between the firstmanifold and the second manifold. The first manifold, second manifoldand tube bank assembly is commonly referred to in the heat exchanger artas a slab. Additionally, a plurality of fins are disposed between eachneighboring pair of heat exchange tubes for increasing heat transferbetween a fluid, commonly air in HVACR applications, flowing over theouter surface of the flattened tubes and along the fin surfaces and afluid, commonly refrigerant in HVACR applications, flowing inside theflattened tubes. Such single tube bank heat exchangers, also known assingle slab heat exchangers, have a pure cross-flow configuration. In anembodiment of flattened tube commonly used in HVACR applications, theinterior of the flattened tube is subdivided into a plurality ofparallel flow channels. Such flattened tubes are commonly referred to inthe art as multichannel tubes, mini-channel tubes or micro-channeltubes.

Double bank flattened tube and fin heat exchangers are also known in theart. Conventional double bank flattened tube and fin heat exchangers,also referred to in the heat exchanger art as double slab heatexchangers, are typically formed of two conventional fin and tube slabs,one disposed behind the other, with fluid communication between themanifolds accomplished through external piping. However, to connect thetwo slabs in fluid flow communication in other than a parallelcross-flow arrangement requires complex external piping. For example,U.S. Pat. No. 6,964,296 shows a flattened tube and fin heat exchanger inboth a single slab and a double slab embodiment with horizontal tuberuns and vertically extending fins. U.S. Patent Application PublicationNo. US 2009/0025914 A1 shows a double slab flatted tube and fin heatexchanger wherein each slab has vertical tube runs extending between apair of horizontally extending manifolds and includes corrugated finsdisposed between adjacent tubes.

A concern associated with the use of flattened tube heat exchangers ascondensers in HVACR applications is poor drainage of retained water fromthe external surface of the flattened tubes and fin matrix. Theretention of water can be particularly problematic in flattened tubeheat exchangers having horizontal tubes with high fin density,sufficient flattened tube depth and close flattened tube spacing commonin condenser applications. In such constructions, water tends to collecton the flat horizontal surfaces of the heat exchange tubes in the spacesbetween the densely packed fins. The water collecting on the externalsurfaces of the heat exchanger tubes acts as an electrolyte and tends toaccelerate corrosion and pitting of the tube surface. Water retention onthe horizontal surface of the heat exchanger tube may also result inincreased airside pressure drop and reduced air flow which adverselyaffects the thermal performance of the system. Any water collecting onthe horizontal tube surface also constitutes a layer of added thermalresistance to heat transfer on the airside of the heat exchange tubes.

Accordingly, the need exists for a flattened tube finned heat exchangerthat is substantially free draining of retained water off the horizontalflat surface of the flattened horizontally extending heat exchangetubes. The desire also exists for a flattened tube finned heat exchangerthat is substantially free draining of water, while also achievingenhanced thermal performance. The need also exists for a double slabflattened tube finned heat exchanger of simplified construction and amethod for assembling the heat exchanger for high volume semi-automatedproduction.

SUMMARY OF THE INVENTION

In an aspect, a multiple slab flattened tube finned heat exchanger isprovided that offers improved drainage of retained water, particularlyin condenser applications, while exhibiting enhanced thermal performanceand reduced risk of failure due to thermal fatigue.

In an aspect, a heat exchanger for transferring heat between a firstfluid and a second fluid includes at least a first heat exchanger slaband a second heat exchanger slab disposed in generally parallelalignment with first heat exchanger slab. Each of the first and secondheat exchanger slabs includes a first manifold, a second manifold spacedfrom the first manifold, and a tube bank including a plurality of tubesegments extending longitudinally in spaced relationship between thefirst manifold and the second manifold and defining a flow passage forthe first fluid. The first manifold of the first heat exchanger slab andthe first manifold of the second heat exchanger slab are juxtaposed inspaced relationship at a first side of the heat exchanger, and thesecond manifold of the second heat exchanger slab and the secondmanifold of the first heat exchanger slab are disposed at a second sideof the heat exchanger. A spacer may be disposed between the firstmanifold of the first heat exchanger slab and the first manifold of thesecond heat exchanger slab for maintaining a desired spacing between thefirst manifolds.

A plurality of folded fins may be disposed in a flow path of the secondfluid defined between the plurality of spaced flattened tube segments ofthe first and second tube banks. Each fold fin has a depth extending atleast from a leading edge of the flattened tube segments of the firsttube bank to a trailing edge of the tube segments of the second tubebank, the second tube bank being disposed aft of the first tube bankwith respect to flow of the second fluid through the flow path of thesecond fluid. In an embodiment, at least one of the folded fins has aleading edge portion that overhangs the leading edge of the first tubebank.

The second heat exchanger slab may be positioned downstream with respectto flow of the second fluid of said first heat exchanger slab whereby agap is provided between a trailing edge of the first tube bank and aleading edge of the second tube bank. In an embodiment, the tubesegments f the first tube bank and the second tube bank are flattenedtube segments. In an embodiment, a ratio the depth of the flattened tubesegments to the depth of the gap is in the range between 1.2 and 6.0,inclusive. In an embodiment, a ratio the depth of the flattened tubesegments to the depth of the gap is in the range between 1.2 and 6.0,inclusive.

In an aspect, a method is provided for adjusting a ratio of the primaryheat transfer surface area collectively defined by the first and secondplurality of flattened tube segments to the secondary heat transfersurface area collectively defined by the plurality of folded fin stripsby increasing or decreasing a depth of the gap.

In an embodiment, the second manifold of the first heat exchanger slabhas at least one flow cutout formed in a side wall thereof and thesecond manifold of the second heat exchanger slab has at least one flowcutout formed in a side wall thereof, and the second manifolds aredisposed in side-by-side relationship with the at least one flow cutoutin the second manifold of the first heat exchanger slab and the at leastone flow cutout in the second manifold of the second heat exchanger slabin registration so as to define a flow passageway for the fluid to becooled to flow from the second manifold of the second heat exchangerslab into the second manifold of the first heat exchanger slab. Inanother embodiment, the second manifold of the first heat exchanger slaband the second manifold of the second heat exchanger slab are formed ina single manifold structure on opposite sides of a common interfacewall. The common interface wall has at least one flow cutout extendingtherethrough defining a flow passageway for the fluid to be cooled toflow from the second manifold of the second heat exchanger slab into thesecond manifold of the first heat exchanger slab.

At least one of the first heat exchanger slab and the second heatexchanger slab may include at least one of the first manifold and secondmanifold thereof being offset from a centerline of the respective tubebank of the first and second tube banks. In an embodiment, at least oneof the first heat exchanger slab and the second heat exchanger slabincludes the first manifold thereof being offset from the centerline ofthe respective tube bank thereof by a first offset distance and thesecond manifold thereof being offset from the centerline of therespective tube bank thereof by a second offset distance, the firstoffset distance and the second offset distance being unequal.

In another aspect, the first manifold of the second heat exchanger slabdefines an inlet header for receiving the fluid to be cooled anddistributing the fluid to be cooled amongst the tubes of the tube bankof the second heat exchanger slab, the second manifold of the secondheat exchanger slab defines an intermediate header for receiving thefluid to be cooled from the tubes of the second tube bank, the secondmanifold of the first heat exchanger slab defines an intermediate headerand an intermediate header for receiving the fluid to be cooled from thesecond manifold of the second heat exchanger slab and distributing thefluid to be cooled to a first number of the tubes of the first tube bankand a separate outlet header for receiving the fluid to be cooled from asecond number of the tubes of the first tube bank, and the firstmanifold of the first heat exchanger slab defines an intermediate headerfor receiving the fluid to be cooled from the first number of the tubesof the first tube bank and for distributing the fluid to be cooledamongst the second number of the tubes of the first tube bank. The firstmanifold of the second heat exchanger slab defines an interior volumehaving a first cross-sectional area, the second manifold of the secondheat exchanger slab defines an interior volume having a secondcross-sectional area, the second manifold of the first heat exchangerslab defines an interior volume having a third cross-sectional area, andthe first manifold of the first heat exchanger slab defines an interiorvolume having a fourth cross-sectional area. The first cross-sectionalarea has the largest magnitude. In an embodiment, the magnitude of thecross-sectional areas decreases successively from the firstcross-sectional area to the fourth cross-sectional area.

In an embodiment, a plurality of flattened tube segments of the secondtube bank collectively define a first flow pass for the fluid to becooled, a first number of a plurality of flattened tube segments of thefirst tube bank collectively define a second flow pass for the fluid tobe cooled, and a second number of the plurality of flattened tubesegments of the first tube bank collectively define a third flow passfor the fluid to be cooled. In an embodiment, the third flow pass isdisposed above the second flow pass within the first tube bank, and thesecond manifold of the first heat exchanger slab defines a lowerintermediate header and an upper outlet header, with the lowerintermediate header in flow communication with the second flow pass andthe upper outlet header in flow communication with the third flow pass.The third first fluid flow pass is positioned on top of the second firstfluid flow path such that the first fluid flows in an overall verticallyupward direction first through the second fluid flow pass and thenthrough the third first fluid flow pass.

In an embodiment where the fluid to be cooled is a refrigerant and thecooling fluid is air, the first refrigerant flow pass comprises arefrigerant desuperheating and condensing pass; the second refrigerantflow pass comprises a refrigerant condensing pass, and the thirdrefrigerant flow pass comprises a refrigerant condensing and subcoolingpass. In a configuration of this embodiment, a ratio of the first numberof flattened tube segments defining the second refrigerant flow pass tothe second number of flattened tube segments defining the thirdrefrigerant flow pass ranges from a 70%/30% split to a 80%/20% split.

In a further aspect, a method is provided for assembling a flattenedtube finned heat exchanger having a first tube bank and a second tubebank. The method includes the steps of: assembling a first plurality offlattened heat exchange tube segments in parallel spaced relationshipwith a continuous folded fin disposed between each pair of parallelflattened heat exchange tube segments to form a partially assembled finand tube pack; providing a first spacer strip and a second spacer strip,each of the first and second spacer strips having a desired depth thatequals a desired value that is the same for both the first and secondspacer strips; positioning the first spacer on a first side of thepartially assembled fin and tube pack and a the second spacer on asecond side of the partially assembled fin and tube pack, each spacerstrip extending transversely across a leading edge of each of theflattened tube segments; inserting a second plurality of flattened heatexchange tubes into the partially assembled fin and tube pack such thata trailing edge of each of inserted tube aligns with a leading edge of arespective one of the first plurality of flattened heat exchange tubesand abuts both the first spacer strip and the second spacer strip toform an assembled fin and tube pack; compressing the assembled fin andtube pack between end braze bars; mounting a first manifold to therespective first ends of each of the first plurality of flattened heatexchange tubes, mounting a second manifold to the respective second endsof the first plurality of flattened heat exchange tubes, mounting athird manifold to the respective first ends of each of the secondplurality of flattened heat exchange tubes, and mounting a fourthmanifold to the respective second ends of the second plurality offlattened heat exchange tubes, thereby forming a final assembly;removing the first and second spacer strips from the final assembly; andbonding the final assembly by brazing in a brazing furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the disclosure, reference will be made tothe following detailed description which is to be read in connectionwith the accompanying drawings, where:

FIG. 1 is a diagrammatic illustration of an exemplary embodiment of amultiple tube bank, flattened tube finned heat exchanger as disclosedherein;

FIG. 2 side elevation view of an exemplary arrangement of a refrigerantcondenser incorporating of pair of multiple tube bank, flattened tubefinned heat exchanger as disclosed herein;

FIG. 3 is a side elevation view, partly in section, illustrating anembodiment of a fin and flattened tube assembly of the heat exchanger ofFIG. 1;

FIG. 4 is a side elevation view, partly in section, illustrating anotherembodiment of a fin and flattened tube assembly of the heat exchanger ofFIG. 1;

FIG. 5 is a top plan view of the heat exchanger of FIG. 1;

FIG. 6 is a plan view, partly in section, illustrating the connectionbetween the heat exchange tubes and the respective manifolds of the heatexchanger of FIG. 1;

FIG. 7 is a perspective view of an embodiment of an one-piece extrudedassembly of the manifolds at the inlet side of the heat exchanger ofFIG. 1;

FIG. 8 is an exploded view of a two piece embodiment of the manifolds atthe refrigerant outlet side of the heat exchanger of FIG. 1;

FIG. 9 is a cross-section view of a single piece embodiment of themanifolds at the refrigerant outlet side of the heat exchanger of FIG.1;

FIG. 10 is an exploded view of another embodiment of the assembly of themanifolds at the refrigerant outlet side of the heat exchanger of FIG.1;

FIG. 11A is a cross-section of another embodiment of the manifolds atthe refrigerant outlet side of the heat exchanger of FIG. 1;

FIG. 11B is a cross-section of another embodiment of the manifolds atthe refrigerant outlet side of the heat exchanger of FIG. 1; and

FIG. 12 is a plan view of a fabrication assembly of the heat exchangerof FIG. 1 assembled in accordance with the method of fabricating amultiple bank flattened tube finned heat exchanger as disclosed herein.

DETAILED DESCRIPTION

There is depicted in FIG. 1 in perspective illustration an exemplaryembodiment of a multiple bank flattened tube finned heat exchanger 10 inaccordance with the disclosure. In FIG. 2 an exemplary embodiment of arefrigerant condenser 20 that includes a pair of multiple bank flattentube finned heat exchangers 10A, 10B, disposed in a generally V-shapedarrangement, and an associated air moving device, for example condenserfan 22 for drawing a flow of a cooling media, for example ambient air,A, through the heat exchangers 10A, 10B, in heat exchange relationshipwith a flow of refrigerant, R, passing through the flattened tubesegments of the heat exchangers 10A, 10B. The lower end of each heatexchanger 10A, 10B is disposed at the bottom of the V-shaped arrangementand the upper end of each heat exchanger 10A, 10B is disposed at top ofthe V-shaped arrangement. As depicted therein, the multiple bankflattened tube finned heat exchanger 10 includes a first heat exchangerslab 10-1 and a second heat exchanger slab 10-2 that is disposed behindthe first tube bank 100, that is downstream with respect to air flow, A,through the heat exchanger 10. It has to be noted that theV-configuration is purely exemplary, and other arrangements arepermissible and within the scope of the invention.

Referring now to FIG. 1, the first heat exchanger slab 10-1 includes afirst manifold 102 extending along a vertical axis, a second manifold104 extending along a vertical axis and spaced apart from the firstmanifold 102, and a first tube bank 100 connecting the first manifold102 and the second manifold 104 in fluid communication and including aplurality of heat exchange tube segments 106, including at least a firstand a second tube segment. In the depicted embodiment, the plurality ofheat exchange tube segments 106 extend longitudinally in spaced parallelrelationship between and connect the first manifold 102 and the secondmanifold 104 in fluid communication. Similarly, the second heatexchanger slab 10-2 includes a first manifold 202 extending along avertical axis, a second manifold 204 also extending along a verticalaxis and spaced apart from the first manifold 202, and a second tubebank 200 connecting the first manifold 202 and the second manifold 204in fluid communication and including a plurality of heat exchange tubesegments 206, including at least a first and a second tube segment. Inthe depicted embodiment, the plurality of heat exchange tube segments206 extend longitudinally in spaced parallel relationship between andconnect the first manifold 202 and the second manifold 204 in fluidcommunication. It is to be understood, however, that one or both of thetube banks 100 and 200 may comprise one or more serpentine tubes havinga plurality of heat exchange tube segments extending in longitudinallyspaced parallel relationship and interconnected by return bends to forma serpentine tube connected at its respective ends between therespective first and second manifolds of the tube banks. As will bediscussed in detail later herein, the first and second heat exchangerslabs 10-1, 10-2 are juxtaposed in generally adjacent relationship withthe first manifold 102 of the first heat exchanger slab 10-1 and thefirst manifold 202 of the second heat exchanger slab 10-2 disposed atthe refrigerant inlet side 12 of the heat exchanger 10 and with thesecond manifold 104 of the first heat exchanger slab 10-1 and the secondmanifold 204 of the second heat exchanger slab 10-2 disposed at therefrigerant outlet side 14 of the heat exchanger 10. It has to be notedthat although a dual slab heat exchanger construction is depicted inFIG. 1, the design can be extended to multiple slabs with no limitation,primarily dictated by economics and available footprint.

Referring now to FIGS. 3-4, in the depicted embodiment, each of the heatexchange tube segments 106, 206 comprises a flattened heat exchange tubehaving a leading edge 108, 208, a trailing edge 110, 210, an upper flatsurface 112, 212, and a lower flat surface 114, 214. The leading edge108, 208 of each heat exchange tube segment 106, 206 is upstream of itsrespective trailing edge 110, 210 with respect to air flow through theheat exchanger 10. In the embodiment depicted in FIG. 3, the respectiveleading and trailing portions of the flattened tube segments 106, 206are rounded thereby providing blunt leading edges 108, 208 and trailingedges 110, 210. In the embodiment depicted in FIG. 4, however, therespective leading and trailing portions of the flattened tube segments106, 206 are tapered to provide knife edge like leading edges 108, 208and trailing edges 110, 210.

The interior flow passage of each of the heat exchange tube segments106, 206 of the first and second tube banks 100, 200, respectively, maybe divided by interior walls into a plurality of discrete flow channels120, 220 that extend longitudinally the length of the tube from an inletend of the tube to the outlet end of the tube and establish fluidcommunication between the respective headers of the first and the secondtube banks 100, 200. In the embodiment of the multi-channel heatexchange tube segments 106, 206 depicted in FIGS. 3 and 4, the heatexchange tube segments 206 of the second tube bank 200 may have agreater width than the heat exchange tube segments 106 of the first tubebank 100. Also, the interior flow passages of the wider heat exchangetube segments 206 is divided into a greater number of discrete flowchannels 220 than the number of discrete flow channels 120 into whichthe interior flow passages of the heat exchange tube segments 106 aredivided.

The second tube bank 200 of the second (rear) heat exchanger slab 10-2,is disposed behind the first tube bank 100 of the first (front) heatexchanger slab 10-1, with respect to the flow of air, A, through theheat exchanger 10, with each heat exchange tube segment 106 directlyaligned with a respective heat exchange tube segment 206 and with theleading edges 208 of the heat exchange tube segments 206 of the secondtube bank 200 spaced from the trailing edges 110 of the heat exchangetube segments of the first tube bank 100 by a desired spacing, G. In theembodiment depicted in FIG. 3, the desired spacing, G, is established byan open gap, thereby providing an open water/condensate drainage spacebetween the trailing edge 110 and the leading edge 208 of each set ofaligned heat exchange tube segments 106, 206 along the entire length ofthe heat exchange tube segments 106, 206. The ratio of the flattenedtube segment depth and gap G is defined by thermal and drainagecharacteristics and positioned between 1.2 and 6.0, with the optimumresiding between 1.5 and 3.0.

The flattened tube finned heat exchanger 10 disclosed herein furtherincludes a plurality of folded fins 320. Each folded fin 320 is formedof a single continuous strip of fin material tightly folded in aribbon-like fashion thereby providing a plurality of closely spaced fins322 that extend generally orthogonal to the flattened heat exchangetubes 106, 206. Typically, the fin density of the closely spaced fins322 of each continuous folded fin 320 may be about 18 to 25 fins perinch, but higher or lower fin densities may also be used. The depth ofeach of the ribbon-like folded fin 320 extends at least from the leadingedge 108 of the first tube bank 100 to the trailing edge of 210 of thesecond bank 200 as illustrated in FIG. 3. Thus, when a folded fin 320 isinstalled between a set of adjacent heat exchange tube segments in theassembled heat exchanger 10, a first section 324 of each fin 322 isdisposed within the first tube bank 100, a second section 326 of eachfin 322 spans the spacing, G, between the trailing edge 110 of the firsttube bank 100 and the leading edge 208 of the second tube bank 200, anda third section 328 of each fin 322 is disposed within the second tubebank 200. In an embodiment, each fin 322 of the folded fin 320 may beprovided with louvers 330, 332 formed in the first and third sections,respectively, of each fin 322. In an embodiment of the flattened tubefinned heat exchanger 10, with respect to the first tube bank 100, theleading portion 336 of each folded fin 320 extends upstream with respectto air flow through air side pass of the heat exchanger 10 so as tooverhang the leading edges 108 of the flattened tube segments 106 of thefirst tube bank 100, for example as illustrated in FIG. 4. The louvercount and louver geometry may be different within each section of thefins 322 and may be related to the respective flattened tube depth. Theratio of the flattened tube segment depth (leading edge to trailingedge) to fin depth (leading edge to trailing) is defined by thermal anddrainage characteristics and in an embodiment is positioned between 0.30and 0.65, inclusive, and in another embodiment resides between 0.34 and0.53, inclusive. Similarly, the ratio of the fin overhang to theflattened tube segment depth is defined by thermal and drainagecharacteristics and ranges between 0 and 0.5, inclusive, and in anembodiment is between 0.13 and 0.33, inclusive.

Heat exchange between the refrigerant flow, R, and air flow, A, occursthrough the outer surfaces 112, 114 and 212, 214, respectively, of theheat exchange tube segments 106, 206, collectively forming the primaryheat exchange surface, and also through the heat exchange surface of thefins 322 of the folded fin 320, which forms the secondary heat exchangesurface. In the multiple bank, flattened tube finned heat exchanger 10disclosed herein, because the fins 322 of the folded fin 320 span thespacing, G, the ratio of the surface area of the primary heat exchangesurface to the surface area provided by the secondary heat exchangesurface may be selectively adjusted without changing the width of thetube segments or the spacing between parallel tube segments. Ratherduring the design process, the depth of the spacing, G, may be increasedto increase the surface area provided by the folded fin 320, therebydecreasing the ratio of primary to secondary heat exchange surface, ormay be decreased to decrease the surface area provided by the folded finplate 320, thereby increasing the ratio of primary to secondary heatexchange surface. The ratio of primary heat exchange surface tosecondary heat exchange surface may also be decreased by increasing theoverall fin depth by increasing the distance by which the leadingportion 336 of the folded fin 320 extends upstream with respect to airflow, A, beyond the face of the heat exchanger 10 and/or by reducing thenumber of flatted tube rows forming the tube banks of both the heatexchanger slabs.

In the embodiment depicted in FIGS. 5 and 6, the neighboring manifolds102 and 202 at the refrigerant inlet side of the heat exchanger 10 areseparate manifolds ends of the tube banks 100, 200 may be separatemanifolds that are maintained in spaced relationship by a spacer 130. Inthe embodiment depicted in FIG. 7, however, the first manifold 102 ofthe first heat exchanger slab 10-1 and the first manifold 202 of thesecond heat exchanger slab 10-2 are formed, for instance extruded andmachined, as a single piece assembly. At the refrigerant outlet side 14of the heat exchanger 10, the neighboring manifolds 104, 204 may becombined, for example brazed together in side-by-side relationship, suchas depicted in FIGS. 6 and 8, or integrated into a single piececonstruction, such as depicted in FIG. 9, that is subdivided by a commoninterface wall 250 into separate manifolds 104, 204 that are fluidlyinterconnected internally within the single piece construction toaccommodate the particular refrigerant circuitry incorporated into theheat exchanger 10. In an embodiment, the neighboring manifolds 104, 204at the refrigerant outlet side of the heat exchanger 10 may be separatemanifolds connected in refrigerant flow communication by flow passage125 defined by at least one flow channel 265 interconnecting themanifolds 104, 204, such as depicted in FIG. 11A or by flow passage 125defined by a least one return bend tube 275 interconnecting themanifolds 104, 204, such as depicted in FIG. 11B. It has to be notedthat the cross-sectional shape of the flow passage 125 can havedifferent configurations such as linear, circular arch-like, ellipticalarc-like, etc. Further, the ratio of the overall collectivecross-sectional area of the flow passages 125 to the cross-section areaof the manifold may be between 0.25 and 2.5, inclusive. In anembodiment, ratio of the overall collective cross-sectional area of theflow passages to the cross-sectional area of the manifold is between 1.0and 1.5, inclusive.

Referring now again to FIG. 1, the multiple tube bank flattened tubefinned heat exchanger 10 will be described as configured as arefrigerant condenser heat exchanger in a refrigerant vapor compressionsystem of an air conditioning unit, transport refrigeration unit orcommercial refrigeration unit, for example, such as but not limited to,the refrigerant condenser 20 depicted in FIG. 2. In such applications,refrigerant vapor (labeled “R”) from the compressor (not shown) of therefrigerant vapor compression system (not shown) passes through themanifolds and heat exchange tube segments of the tube banks 100, 200 ofthe first and second heat exchanger slabs 10-1, 10-2, in a manner to bedescribed in further detail hereinafter, in heat exchange relationshipwith a cooling media, most commonly ambient air, flowing through theairside of the heat exchanger 10 in the direction indicated by the arrowlabeled “A” that passes over the outside surfaces of the heat exchangetube segments 106, 206, commonly referred to as the primary heatexchange surface, and the surfaces of the corrugated fins 320, commonlyreferred to as the secondary heat exchange surface.

The multiple tube bank flattened tube finned heat exchanger 10 depictedin FIG. 1 has a cross-counterflow circuit arrangement. The air flowfirst passes transversely across the upper and lower horizontal surfaces112, 114 of the heat exchange tube segments 106 of the first tube bank,and then passes transversely across the upper and lower horizontalsurfaces 212, 214 of the heat exchange tube segments 206 of the secondtube bank 200. The refrigerant passes in cross-counterflow arrangementto the airflow, in that the refrigerant flow passes first through thesecond tube bank 200 and then through the first tube bank 100. In theprocess, the refrigerant passing through the flow channels of the heatexchange tubes 106, 206 rejects heat into the airflow passing throughthe air side of the heat exchanger 10. The multiple tube bank flattenedtube finned heat exchanger 10 having a cross-counterflow circuitarrangement yields superior thermal performance, as compared to the purecrossflow or cross-parallel flow circuit arrangements. As stated above,the invention is not limited to the dual slab configurations and can bereadily extended to arrangements with more the two slabs.

More specifically, in the embodiment depicted in FIG. 1, the refrigerantflow, designated by the label “R”, passes from the refrigerant circuit(not shown) into the first manifold 202 of the second heat exchangerslab 10-2, via refrigerant inlets 222 (FIG. 6), and is distributedamongst all the heat exchange tube segments 206 of the second tube bank200, collectively forming a first refrigerant pass 401 through the heatexchanger 10, to flow therethrough into the second manifold 204 of thesecond heat exchanger slab 10-2. The refrigerant collecting in thesecond manifold 204 of the second heat exchanger slab 10-2 then passesinternally into a lower section 116 of the second manifold 104 of thefirst heat exchanger slab 10-1 and is distributed amongst a firstportion of the heat exchange tube segments 106 of the first tube bank100. A flow impervious baffle 115 is disposed across the interior volumeof the second manifold 104 of the first heat exchanger slab 10-1 so asto divide the interior volume into a lower volume in lower section 116of the second manifold 104 and an upper volume in upper section 118 ofthe second manifold 104. The refrigerant passes from the lower section116 of the second manifold 104 through the first portion of theflattened tube segments 106 collectively forming a second refrigerantpass 402 through a lower portion of the first tube bank 100 and into thefirst manifold 102 of the first heat exchanger slab 10-1. From the firstmanifold 102 the refrigerant is distributed amongst a second portion ofthe heat exchange tube segments 106 of the first tube bank 100,collectively forming a third refrigerant pass 403 through the heatexchanger 10 through an upper region of the first tube bank 100, to flowtherethrough into the upper portion 118 of the second manifold 104 ofthe first tube bank 100. The refrigerant passes from the second manifold104 of the first tube bank 100 through refrigerant outlet 122 (FIG. 6)back into the refrigerant circuit of the refrigerant vapor compressionsystem (not shown).

The flow impervious baffle plate 115 is disposed across the interiorvolume of the second manifold 104 to divide the interior volume of thesecond manifold 104 into the lower portion 116 that serves as anintermediate header and the upper portion 118 that serves as an outletheader. During manufacture of the second manifold 104 of the first heatexchanger slab 10-1, the baffle plate 115 may be positioned as desiredwithin the interior volume of the second manifold 104 to select adesired split with respect to the number of heat exchange tube segment106 forming the second refrigerant pass 402 and the number of heatexchange tube segments 106 forming the third refrigerant pass 403.

In a refrigerant condenser application, the baffle plate 115 can beselectively positioned such that the split between the number of heatexchange tubes 106 within the second refrigerant pass and the number ofheat exchange tubes 106 within the third refrigerant pass is in therange from a 70%/30% split to an 80%/20% split. Thus, the split innumber of heat exchange tube segments between the second refrigerantpass and the third refrigerant pass may be selected to controlrefrigerant pressure drop through the heat exchanger and/or to reducerefrigerant maldistibution amongst the heat exchange tube segments.Additionally, the respective interior volumes of the manifolds 102, 104,202 and 204 need not be the same, but may vary to compensate for achange in density of the refrigerant flowing through the heat exchanger10 and/or once again, control refrigerant distribution. In refrigerantcondenser applications, for example, the cross-sectional area of thefirst manifold 202 of the second heat exchanger slab 10-2 on the inletside 12 of the heat exchange 10, which receives incoming refrigerantvapor from a refrigerant circuit, could have a larger cross-sectionalarea than the second manifold 204 of the second heat exchanger slab 10-2on the outlet side 14 of the heat exchanger 10, which receives a cooledand generally partially condensed refrigerant vapor/liquid mixturehaving traversed the first pass 401 of the heat exchanger 10. The firstmanifold 202 of the second heat exchanger slab 10-2 could also have alarger cross-sectional area than the first manifold 102 of the firstheat exchanger slab 10-1, which is also disposed on the inlet side 12 ofthe heat exchanger 10, but receives a refrigerant vapor/liquid mixturefrom the second refrigerant pass 402 which is predominately condensedliquid.

Thus, the refrigerant circuit of the embodiment of the multiple bankheat exchanger hereinbefore described and depicted in FIG. 1 is a singlepass, two pass up-flow, cross-counterflow refrigerant circuit. In therefrigerant condenser arrangement as described hereinbefore, the firstrefrigeration pass 401 functions in part as a desuperheating andcondensing pass, the second refrigerant pass 402 as a condensing pass,and the third refrigerant pass 403 as a condensing and subcooling pass.The up-flow circuit arrangement of the second refrigerant pass 402 tothe third refrigerant pass 403 ensures the any refrigerant vaporsurviving passage through the second refrigerant pass 402 flows upwardlyagainst gravity through a column of refrigerant liquid within the firstmanifold 102. With this up-flow circuit arrangement, better mixing ofrefrigerant vapor and liquid flowing through the second refrigerant pass402 is expected to enhance tube-side heat transfer. Also, the vaporrefrigerant is typically exposed to a higher airflow in the flattenedtubes of the second pass positioned closer to the air moving device,thus enhancing the condensation process. With the third refrigerant pass403 in the upper region of the first tube bank 100, the refrigerantsubcooling regions of the heat exchangers 10A and 10B are positioned inthe upper region of the refrigerant condenser heat exchanger 20 whichtends to be even higher air flow region of the heat exchanger 20 asshown in FIG. 2. As the collected water on the surfaces of the heatexchange tube segments 106, 206 and the surfaces of the fins 322 of thecontinuous folded fins 320 tends to remain longer within the region ofthe subcooling due to lower temperatures, the higher air flow throughthe upper region of the heat exchanger 20 will assist in removing theaccumulating water on the heat exchange surfaces, as well as the gravityassisted water drainage is expected to dry out the upper portion of theheat exchanger faster, especially during the prolonged unit shutdownperiods. It has to be noted that more than a single pass arrangement forthe back slab (with respect to the airflow) and more than two passarrangement for the first slab (with respect to the airflow) are withinthe scope of the invention, as long as a subcooling pass is located inthe upper portion of the heat exchanger.

As noted previously, the second tube bank 200, i.e. the rear heatexchanger slab, is disposed behind the first tube bank 100, i.e. thefront heat exchanger slab. As best seen in FIGS. 6 and 7, the respectivesecond manifolds 104 and 204 of the heat exchanger 10 are disposed incontact with each other, but the respective first manifolds 102 and 202of the heat exchanger 10 are disposed in spaced relationship relative toeach other. As described previously, in the heat exchanger 10 disclosedherein, the first manifold 202 of the second heat exchanger slab 10-2defines an inlet header that receives high temperature refrigerant vaporfrom a refrigerant circuit through at least one refrigerant inlet 222and distributing the received refrigerant amongst the heat exchange tubesegments 206 forming the second tube bank 200; the second manifold 204of the second heat exchanger slab 10-2 defines an intermediate headerfor receiving refrigerant having traversed the heat exchange tubesegments 206; the second manifold 104 of the first heat exchanger slab10-1 defines both a lower intermediate header for distributingrefrigerant amongst the heat exchange tube segments 106 in a lowerportion of the first tube bank 100 and an upper outlet header in fluidcommunication with the refrigerant circuit through refrigerant outlet122 for returning subcooled refrigerant liquid to the refrigerantcircuit; and the first manifold 102 of the first heat exchanger slab10-1 defines an intermediate header providing fluid flow communicationbetween the heat exchange tube segments 106 forming the lower pass ofthe first tube bank 100 and the heat exchange tube segments 106 formingthe upper pass of the first tube bank 100. An impervious flow baffle 115is disposed across the internal volume of the second manifold 104 at aselected location along the longitudinal extent of the second manifold104, thereby dividing the interior volume defined by the second manifold104 into the lower intermediate header and the upper outlet header.

With this refrigerant flow arrangement, the temperature differentialbetween the high temperature refrigerant vapor received in the inletheader of the first manifold 202 of the second tube bank 200 and thecooler refrigerant, which may be a mix of liquid and vapor, flowingthrough the intermediate header of the first manifold 102 of the firsttube bank 100 results in uneven thermal expansion with respect to thefirst manifold 102 and the first manifold 202. Therefore, in the heatexchanger 10 as disclosed herein, provision is made to allow uneventhermal expansion to occur between the neighboring manifolds 102, 202 atthe refrigerant inlet side of the heat exchanger 10.

In an embodiment, such as depicted in FIG. 6, at least one spacer 130 isdisposed between the first manifold 102 and the first manifold 202 ateach end cap of the manifolds and at the center to maintain spacingbetween the first manifold 102 and the first manifold 202 whereby theonly contact between the first manifold 102 and the first manifold 202is through the spacer 130 which is fixed, for example brazed or welded,to only one of the inlet manifolds 102, 202. On the contrary, the spacercan be removed after final multiple slab heat exchanger assembly orafter the brazing process. In the latter case the spacer for instancecan be made from steel or other suitable material that does not braze.In an embodiment, the spacer 130 may comprise a generally U-shaped tab,x-shaped tab or an end cap. In an embodiment, the spacer 130 may be anelongated graphite sheet disposed between the first manifold 102 and thefirst manifold 202. By maintaining a space between the first manifold102 and the first manifold 202 for minimizing contact therebetween,thermal stress as a result of the aforementioned uneven thermalexpansion is avoided and thermal cross conduction from the inletmanifold 202 to the inlet manifold 102 is minimized. Conventionalmultiple slab heat exchanger constructions, however, characteristicallyexhibit a higher risk of failure due to thermal fatigue as a result ofthe uneven thermal expansion. In an embodiment, the space maintainedbetween the manifolds 102 and 202 by the spacer tabs 130 typicallyranges from 0.5 to 8 millimeters.

In the embodiment depicted in FIG. 7, however, the first manifold 102 ofthe first heat exchanger slab 10-1 and the first manifold 202 of thesecond heat exchanger slab 10-2 are formed together as a single-piecemanifold assembly 160. The single piece manifold assembly 160 isfabricated (e.g. extruded) as two elongated generally tubular membersdisposed is spaced side-by-side relationship with a spacer bar 162bridging the space between and formed integrally with the manifolds 102,202. Upon completion of the extrusion process, the manifold assembly 160is cut to the desired length. At this point, the spacer bar 162 extendsthe full longitudinal length of the manifold assembly. To allow foruneven thermal expansion without excessive thermal stress a portion 164of the spacer bar 162 is machined away longitudinally inwardly at bothends of the manifold assembly 160 leaving the respective end portions ofeach of the first manifold 102, 202 free to expand in a longitudinal andtransverse directions differentially from the other first manifold,thereby minimizing the thermal stress and thermal fatigue that wouldotherwise be experienced as a result of uneven thermal expansion.

Referring again to FIG. 6, the spaced manifolds 102, 202 at therefrigerant inlet side 12 of the heat exchanger 10 are not aligned withthe mated manifolds 104, 204 at the refrigerant outlet side 14 of theheat exchanger 10. In the multiple bank flattened tube finned heatexchanger disclosed herein, the slots provided in each of the manifolds102, 202, 104, 204 for receiving the ends of the heat exchange tubesegments 106, 206 are not centered on the centerlines of the manifoldsas in conventional flattened tube heat exchangers. Rather, asillustrated in FIG. 6, the slots are offset from the centerlines ofrespective manifolds 102, 202, 104, 204 by an amount necessary to ensurethat the heat exchange tube segments 106 of the first tube bank 100 andthe heat exchange tube segments 206 are parallel to each other.

In the embodiment depicted in FIG. 6, the manifolds 202 and 204 are onlyslightly offset from each other and the centerline of the tube segment206. However, the manifolds 102 and 104 of the first heat exchanger slab10-1 are well offset from the centerline 405 of the flattened tubesegment 106. The centerline 407 of the first manifold 102 is offset fromthe centerline 405 of the flattened tube segment 106 by a first offsetdistance 406, which provides a first clearance 408 between the trailingedge 110 and the inside wall of the first manifold 102. However, thecenterline 409 of the second manifold 104 is offset from the centerline405 of the flatted tube segment 106 by a second offset distance 410,which provides a second clearance 412 between the trailing edge of thetube segment 106 and the inside wall of the second manifold 104. Asillustrated in FIG. 7, the first and second offset distances 406, 410are not equal and the first and second clearances are not equal. Rather,the first offset distance 406 is greater than the second offset distance410 and the first clearance 408 is smaller than the second clearance412. For both of the first and second clearances 406, 410, the minimumclearance should be at least 0.75 millimeters.

In the embodiment depicted in FIG. 8, the second manifold 104 of thefirst heat exchanger slab 10-1 and the second manifold 204 of the secondheat exchanger slab 10-2 are disposed in side wall-to-side wall contactalong their length. As illustrated in FIG. 8, a plurality of openings125 are provided at spaced intervals in the side wall of the secondmanifold 104 that interfaces with the side wall of the second manifold104. The openings 125, each of which extends through the side wall ofthe second manifold 104, are confined to the portion of the secondmanifold 104 that defines the intermediate header 116 of the first tubemanifold 100. Similarly, a plurality of openings 225, each of whichextends through the side wall of the second manifold 204, are providedat spaced intervals in the lower portion of the side wall of the secondmanifold 200 that interfaces with the side wall of the first manifold100. The plurality of openings 125 and the plurality of openings 225 areequal in number and are spaced apart such that each opening 125 lies inregistration with a respective one of the openings 225 and therebydefine a plurality of flow passages through which refrigerant may flowfrom the second manifold 204 of the second heat exchanger slab 10-2 (therear heat exchanger slab) into the intermediate header of the firstmanifold 104 of the first heat exchanger slab 10-1 (the front heatexchanger slab). Refrigerant flow communication openings 125 can be ofcircular, oval, racetrack, rectangular or any other suitable shape andshould have size and separation to provide adequate cross-section areafor refrigerant flow and sufficient structural integrity.

Referring now to FIG. 9, there is depicted in cross-section anembodiment of the heat exchanger 10 wherein the second manifold 104 ofthe first heat exchanger slab 10-1 and the second manifold 204 of thesecond heat exchanger slab 10-2 are formed in a single piececonstruction 250 and share a common interface wall 250. A plurality offlow passage cutouts 255 are provided at spaced intervals in the commoninterface wall 252. The flow passage cutouts 255 are confined to thatportion of the common interface wall 250 that bounds the intermediateheader portion of the second manifold 104 of the first tube manifold100. Like the flow passages formed by the registration of the cutouts125 and the cutouts 225 of the interfacing side walls of the secondmanifold 104 and the second manifold 204 in the embodiment depicted inFIG. 8, the flow passage cutouts 255 in the common interface wall 252 ofthe single piece manifold construction 250 depicted in FIG. 9 define aplurality of flow passages through which refrigerant may flow from thesecond manifold 204 of the second tube bank 200 (the rear heat exchangerslab) into the intermediate header of the first manifold 104 of thefirst tube bank 100 (the front heat exchanger slab). In either of thedepicted embodiments, the cutouts 125, 225, 255 may have a width in therange of 20% to 50% of the flat platform of the wall in which thecutouts are provided and the cutouts 125, 225, 255 may have collectiveheight in the range of 20% to 80% of the height of the portion of thesecond manifold 104 that defines the intermediate header 116 of thefirst tube manifold 100. As mentioned above, the cross-sectional shapeof the cutouts 125, 225, 255 can be round, oval, rectangular, square orany other desired configuration.

An exploded view illustrating a method for manufacturing thesingle-piece manifold construction 250 is depicted in FIG. 10. Alongitudinally elongated double barreled tubular member 254 is extrudedand cut at a desired length. Spacer webs 256, 257 extend between the topand bottom sides of the longitudinally extending barrels 258, 259,respectively. Each of the spacer webs 256, 257 includes a pair of spacedtabs 260 integrally formed therewith during extrusion or otherfabrication and extending inwardly along the entire longitudinal extentof the tubular member 254. A wall insert 262 is extruded, or otherfabricated, to have a length equal to the length of the tubular member254, and a height and width to accommodate a sliding fit of the wallinsert 262 into the space defined between the spacer webs 256, 257 andbounded by the tabs 260. To form the center wall 252 dividing the singlepiece double barreled construction 250 into separate second manifolds104, 204, the wall insert 262 and the facing surfaces of the spacer webs256, 257 and tabs 260 are cladded with a brazing compound, the wallinsert 262 (that also may be cladded) is slid into the space between thetabs 260 until fully received into the tubular member 254, and theentire assembly is then heated in a brazing furnace. For a tubularmember 254 made from aluminum or aluminum alloy, the wall insert 262would also be made from aluminum or compatible aluminum alloy. Toaccommodate fluid flow from the second manifold 204 into the secondmanifold 204, the flow cutouts 255 are machined into a selected portionof the wall insert 262 prior to positioning the wall insert 262 in thetubular member 254. Once again, the shape, number and size of thecutouts 255 are defined by the same considerations as specified above.

In the embodiments of the multiple bank flattened tube finned heatexchanger 10 as disposed herein, the manifolds, heat exchange tubes andfins are all made of aluminum or aluminum alloy material. For an allaluminum heat exchanger design, the entire multiple bank flattened tubefinned heat exchanger is assembled and the placed in a brazing furnacewherein the components of the assembled heat exchanger are bonded bybrazing. Referring now to FIG. 12, as well as FIGS. 3 and 4, infabricating the multiple bank flattened tube finned heat exchanger 10,the plurality of tube segments 206 of the second heat exchanger slab10-2 (the rear heat exchanger slab) are assembled in parallel spacedrelationship with a ribbon-like folded fin 20 disposed between each pairof adjacent parallel tube segments 206. As the ribbon-like folded fin 20has a depth sufficient to extend from the leading edge 108 of the tubesegments 106 of the first tube bank 100 of the front heat exchanger slab10-1 to the trailing edge 210 of the tube segments 206 of the secondtube bank 200 of the rear heat exchanger slab 10-2 when the heatexchanger 10 is assembled, a forward portion of each of the folded fins20 may be designed to extend forwardly beyond the leading edges 208 ofthe assembled array of tube segments 206.

The exemplary manufacturing processes to fabricate multiple slabflattened tube finned heat exchanger are described below. It has to beunderstood that these manufacturing processes are provided forillustrative purposes only, and various deviations or alterations ofsuch manufacturing processes may be made without departing from thescope of the method for fabricating a heat exchanger as set forth in theclaims. A side spacer strip is positioned against the tube segment 206outside the folded fin 20 at each end of the assembled array of tubesegments 206 and folded fin 20. The side spacer strips extend from theuppermost tube segment to the lowermost tube segment of the assembledarray of tube segments 206 transversely across and against the leadingedge 208 of each tube segment 206. Each side spacer strip has a depth(thickness) that equals the desired spacing, G, between the trailingedge 110 of the tube segments 106 and the leading edge 208 of the tubesegments 210.

With the side spacer strips so positioned, a tube segment 106 isinserted into each of the respective spaces between the forwardlyextending portions of the folded fin 20 with the trailing edge 110 ofeach tube segment 106 abutting against the side spacer strips at eachend of the tube segment. In this manner, each tube segment 106 isassembled in alignment with a respective one of the tube segments 206.Then assembled folded fin and tube matrix so assembled is nextcompressed between end braze bars and held together by dedicated fixtureclips.

The four manifolds 102, 104, 202 and 204 are now mounted on the tubesegments 106, 206. The manifold 102 has a plurality of openings adaptedto receive the respective ends of the tube segments 106 at an endthereof at the refrigerant inlet side of the heat exchanger 10 and themanifold 104 has a plurality of openings adapted to receive therespective ends of the tube segments 106 at the other end thereof at therefrigerant outlet side of the heat exchanger 10. Similarly, themanifold 202 has a plurality of openings adapted to receive therespective ends of the tube segments 206 at an end thereof at therefrigerant inlet side of the heat exchanger 10 and the manifold 204 hasa plurality of openings adapted to receive the respective ends of thetube segments 206 at the other end thereof at the refrigerant outletside of the heat exchanger 10. The manifolds 104 and 204 are now weldedtogether, side wall to side wall with the respective openings 125 and225 in registration, for example by tack welding.

As noted previously, the manifolds 102 and 202 are not welded together,but rather spaced apart by spacer tabs 130. The desired spacing betweenthe manifolds 102 and 202 is established by tab extensions integral tothe respective end caps at each end of the manifolds 102, 202. In anembodiment, the spacer tabs 130 are formed by bending the tab extensionsinto U-shaped members protruding outwardly from the respective end capsa distance equal to the desired spacing to be maintained between themanifolds 104, 204. The spacer tabs 130 can be coated with a materialthat prevents brazing of the spacer tabs 130 with the abutting manifold.In an alternate embodiment, instead of providing spacer tabs, the aspacer sheet, such as for example a graphite sheet, having a thicknessequal to the desired spacing between the manifolds 102 and 202 andextending substantially the full length of the manifolds may be disposedbetween the manifolds 102 and 202 to maintain the desired spacingbetween the manifolds 102 and 202.

With the manifolds 102, 104, 202, 204 assembled to the stacked array oftube segments 106, 206 and folded fin plates 20, the side spacer stripsare removed and the entire assembly is placed in a brazing furnace. Inthe brazing furnace, each of the folded fins 20 is bonded by brazing tothe respective tube segments 106, 206 against which it abuts.Simultaneously, the manifolds 102, 104 are bonded by brazing to the tubesegments 106 and the manifolds 202, 204 are also bonded by brazing tothe tube segments 206.

While the present invention has been particularly shown and describedwith reference to the exemplary embodiments as illustrated in thedrawing, it will be recognized by those skilled in the art that variousmodifications may be made without departing from the spirit and scope ofthe invention. For example, it is to be understood that the multiplebank flattened tube finned heat exchanger 10 disclosed herein mayinclude more than two tube banks. It is also to be understood that thetube banks 100, 200 could include serpentine tubes with the heatexchange tube segments 106, 206 being parallel linear tube segmentsconnected by U-bends or hairpin turns to form a serpentine tubeconnected at its respective ends between the first manifold and thesecond manifold of the heat exchanger slab. Further, although themultiple tube bank heat exchanger disclosed herein is depicted havingflattened tube segments, various aspects of the invention may be appliedto multiple bank heat exchangers having round tubes or other forms ofnon-round tubes. Therefore, it is intended that the present disclosurenot be limited to the particular embodiment(s) disclosed as, but thatthe disclosure will include all embodiments falling within the scope ofthe appended claims.

We claim:
 1. A heat exchanger for transferring heat between a firstfluid and a second fluid comprising: at least a first heat exchangerslab and a second heat exchanger slab; the first heat exchanger slabincluding a first manifold, a second manifold spaced from the firstmanifold, and a first tube bank extending between the first manifold andthe second manifold and including a plurality of tube segments extendinglongitudinally in spaced relationship, each tube segment defining aninternal flow passage for the first fluid; the second heat exchangerslab including a first manifold, a second manifold spaced from the firstmanifold, and a second tube bank extending between the first manifoldand the second manifold and including a plurality of tube segmentsextending longitudinally in spaced relationship, each tube segmentdefining an internal flow passage for the first fluid, said second heatexchanger slab being disposed in generally parallel alignment with saidfirst heat exchanger slab, the first manifold of said first heatexchanger slab and the first manifold of said second heat exchanger slabjuxtaposed in spaced relationship at a first side of said heatexchanger, and the second manifold of said second heat exchanger slaband the second manifold of said first heat exchanger slab at a secondside of said heat exchanger; and a plurality of folded fins disposed ina flow path of the second fluid through said heat exchanger and inoperative association with the plurality of tube segments of the firsttube bank and the second tube bank, each fold fin having a depthextending at least from a leading edge of the tube segments of the firsttube bank to a trailing edge of the tube segments of the second tubebank, the second tube bank disposed aft of the first tube bank withrespect to flow of the second fluid through the second flow path.
 2. Theheat exchanger as recited in claim 1 further comprising a spacerdisposed between the first manifold of said first heat exchanger slaband the first manifold of said second heat exchanger slab formaintaining a desired spacing between the first manifolds.
 3. The heatexchanger as recited in claim 2 wherein the second manifold of thesecond heat exchanger slab and the second manifold of the first heatexchanger slab are connected in fluid flow communication with respect tothe first fluid.
 4. The heat exchanger as recited in claim 3 wherein thesecond manifold of said first heat exchanger slab has at least one flowcutout formed in a side wall thereof and the second manifold of saidsecond heat exchanger slab has at least one flow cutout formed in a sidewall thereof, the second manifolds disposed in side-by-side relationshipwith the at least one flow cutout in the second manifold of said firstheat exchanger slab and the at least one flow cutout in the secondmanifold of said second heat exchanger slab in registration so as todefine a flow passageway for the first fluid to flow from the secondmanifold of said second heat exchanger slab into the second manifold ofsaid first heat exchanger slab.
 5. The heat exchanger slab as recited inclaim 3 wherein the second manifold of said first heat exchanger slaband the second manifold of said second heat exchanger slab are formed ina single manifold structure on opposite sides of a common interfacewall, the common interface wall having at least one flow cutoutextending therethrough define a flow passageway for the first fluid toflow from the second manifold of said second heat exchanger slab intothe second manifold of said first heat exchanger slab.
 6. The heatexchanger slab as recited in claim 3 wherein the second manifold of saidfirst heat exchanger slab and the second manifold of said second heatexchanger slab are formed as separate manifolds and are connected inflow communication by at least one conduit defining a first fluid flowpassage.
 7. The heat exchanger as recited in claim 1 wherein at leastone of said first heat exchanger slab and said second heat exchangerslab includes at least one of the first manifold and second manifoldthereof being offset from a centerline of the respective tube bank ofthe first and second tube banks.
 8. The heat exchanger as recited inclaim 7 wherein at least one of said first heat exchanger slab and saidsecond heat exchanger slab includes the first manifold thereof beingoffset from the centerline of the respective tube bank thereof by afirst offset distance and the second manifold thereof being offset fromthe centerline of the respective tube bank thereof by a second offsetdistance, the first offset distance and the second offset distance beingunequal.
 9. The heat exchanger as recited in claim 1 wherein each of thetube segments of the first tube bank and the second tube banks comprisesa flattened tube segment having a depth in the direction of flow of thesecond fluid through said heat exchanger and a ratio of the flattenedtube segment depth to the depth of said folded fin is in the rangebetween 0.30 and 0.65.
 10. The heat exchanger as recited in claim 9wherein the at least one of said folded fins has a leading edge portionthat overhangs the leading edge of the first tube bank by a distance upto 0.5 of the depth of the flattened tube segments of the first tubebank.
 11. The heat exchanger as recited in claim 1 wherein at least oneof said folded fins has a leading edge portion that overhangs theleading edge of the first tube bank.
 12. The heat exchanger as recitedin claim 1 wherein said second heat exchanger slab is positioneddownstream with respect to flow of the second fluid of said first heatexchanger slab whereby a gap is provided between a trailing edge of thefirst tube bank and a leading edge of the second tube bank.
 13. The heatexchanger as recited in claim 9 wherein said second heat exchanger slabis positioned downstream with respect to flow of the second fluid ofsaid first heat exchanger slab whereby a gap is provided between atrailing edge of the first tube bank and a leading edge of the secondtube bank, a ratio the depth of the flattened tube segments to the depthof the gap is in the range between 1.2 and 6.0, inclusive.
 14. The heatexchanger as recited in claim 12 wherein a ratio the depth of theflattened tube segments to the depth of the gap is in the range between1.5 and 3.0, inclusive.
 15. A heat exchanger for transferring heatbetween a fluid to be cooled and a cooling fluid comprising: a firstheat exchanger slab including a first manifold, a second manifold spacedfrom the first manifold, and a first tube bank extending between thefirst manifold and the second manifold and including a plurality offlattened tube segments extending longitudinally in spaced parallelrelationship and defining a flow passage for the fluid to be cooled; anda second heat exchanger slab including a first manifold, a secondmanifold spaced from the first manifold, and a second tube bankextending between the first manifold and the second manifold andincluding a plurality of flattened tube segments extendinglongitudinally in spaced parallel relationship and defining a flowpassage for the fluid to be cooled, said second heat exchanger slabdisposed downstream of said first heat exchanger slab, the secondmanifold of said second heat exchanger slab being connected in fluidflow communication with the second manifold of said first heat exchangerslab; wherein: the first manifold of said second heat exchanger slabdefines an inlet header for receiving the fluid to be cooled anddistributing the fluid to be cooled amongst the flattened tube segmentsof the second tube bank; the second manifold of said second heatexchanger slab defines an intermediate header for receiving the fluid tobe cooled from the flattened tube segments of the second tube bank; thesecond manifold of said first heat exchanger slab defines anintermediate header and a separate outlet header, the intermediateheader for receiving the fluid to be cooled from the second manifold ofthe second heat exchanger slab and distributing the fluid to be cooledto a first number of the flattened tube segments of the first tube bank,the outlet header for receiving the fluid to be cooled from a secondnumber of the flattened tube segments of the first tube bank; and thefirst manifold of said first heat exchanger slab defines an intermediateheader for receiving the fluid to be cooled from the first number of theflattened tube segments of the first tube bank and for distributing thefluid to be cooled amongst the second number of the flattened tubesegments of the first tube bank.
 16. The heat exchanger as recited inclaim 15 wherein the plurality of flattened tube segments of the secondtube bank collectively define a first flow pass for the fluid to becooled, the first number of the plurality of flattened tube segments ofthe first tube bank collectively define a second flow pass for the fluidto be cooled, and the second number of the plurality of flattened tubesegments of the first tube bank collectively define a third flow passfor the fluid to be cooled.
 17. The heat exchanger as recited in claim16 wherein: the third flow pass is disposed above the second flow passwithin the first tube bank; and the second manifold of said first heatexchanger slab defines a lower intermediate header and an upper outletheader, the lower intermediate header in flow communication with thesecond flow pass and the upper outlet header in flow communication withthe third flow pass.
 18. The heat exchanger as recited in claim 17wherein the first fluid flow pass comprises at least a refrigerantdesuperheating pass, the second fluid flow pass comprises a refrigerantcondensing pass, and the third fluid flow pass comprises at least arefrigerant subcooling pass.
 19. The heat exchanger as recited in claim17 wherein the first fluid flow pass comprises a refrigerantdesuperheating and condensing pass; the second fluid flow pass comprisesa refrigerant condensing pass, and the third fluid flow pass comprises arefrigerant condensing and subcooling pass.
 20. The heat exchanger asrecited in claim 18 wherein a ratio of the first number of flattenedtube segments defining the second refrigerant flow pass to the secondnumber of flattened tube segments defining the third refrigerant flowpass ranges from a 70%/30% split to a 80%/20% split.
 21. The heatexchanger as recited in claim 17 wherein the second manifold of thesecond heat exchanger slab is connected internally directly in fluidflow communication with the lower intermediate header only of the secondmanifold of the first heat exchanger slab.
 22. The heat exchanger asrecited in claim 17 wherein the second manifold of the second heatexchanger slab is connected by at least one return bend tube in fluidflow communication with the lower intermediate header only of the secondmanifold of the first heat exchanger slab.
 23. The heat exchanger asrecited in claim 15 wherein the first manifold of said second heatexchanger slab defines an interior volume having a first cross-sectionalarea, the second manifold of said second heat exchanger slab defines aninterior volume having a second cross-sectional area, the secondmanifold of said first heat exchanger slab defines an interior volumehaving a third cross-sectional area, and the first manifold of saidfirst heat exchanger slab defines an interior volume having a fourthcross-sectional area, the first cross-sectional area being greater thanfourth the cross-sectional area.
 24. The heat exchanger as recited inclaim 23 wherein the first cross-sectional area has the largestmagnitude and the magnitude of the cross-sectional areas decreasessuccessively from the first cross-sectional area to the fourthcross-sectional area.
 25. A method for fabricating a flattened tubefinned heat exchanger having a first tube bank and a second tube bank,the method comprising the steps of: assembling a first plurality offlattened heat exchange tube segments in parallel spaced relationshipwith a continuous folded fin plate disposed between each pair ofparallel flattened heat exchange tube segments to form a partiallyassembled fin and tube pack; providing a first spacer strip and a secondspacer strip, each of the first and second spacer strips having adesired depth that equals a desired value that is the same for both thefirst and second spacer strips; positioning the first spacer on a firstside of the partially assembled fin and tube pack and the second spaceron a second side of the partially assembled fin and tube pack, eachspacer strip extending transversely across a leading edge of each of theflattened tube segments; inserting a second plurality of flattened heatexchange tubes into the partially assembled fin and tube pack such thata trailing edge of each of inserted tube aligns with a leading edge of arespective one of the first plurality of flattened heat exchange tubesand abuts both the first spacer strip and the second spacer strip toform an assembled fin and tube pack; compressing the assembled fin andtube pack between end braze bars; mounting a first manifold to therespective first ends of each of the first plurality of flattened heatexchange tubes, mounting a second manifold to the respective second endsof the first plurality of flattened heat exchange tubes, mounting athird manifold to the respective first ends of each of the secondplurality of flattened heat exchange tubes, and mounting a fourthmanifold to the respective second ends of the second plurality offlattened heat exchange tubes, thereby forming a final assembly;removing the first and second spacer strips from the final assembly; andbonding the final assembly by brazing in a brazing furnace.
 26. Themethod as recited in claim 25 further comprising the step of tackwelding the second manifold and the fourth manifold together prior tobrazing.
 27. The method as recited in claim 25 further comprisingproviding at least one spacer between the first manifold and the fourthmanifold.
 28. The method as recited in claim 27 wherein the at least onespacer comprises at least two spacer tabs disposed at opposite ends ofthe first and fourth manifolds.
 29. The method as recited in claim 27wherein the at least one spacer comprises an elongated graphite sheetdisposed between the first manifold and the fourth manifold.
 30. In amultiple slab heat exchanger having: a first heat exchanger slab havinga first plurality of flattened tube segments extending longitudinally inspaced parallel relationship to form a first tube bank; a second heatexchanger slab having a second plurality of flattened tube segmentsextending longitudinally in spaced parallel relationship to form asecond tube bank, said second tube bank disposed in generally parallelalignment with said first tube bank with a longitudinal edge of theplurality of flattened tube segments of said second tube bank disposedat a spacing gap from a longitudinal edge of the plurality of flattenedtube segments of said first tube bank; and a plurality of continuousfolded fins extending between the first plurality of flattened tubesegments of said first tube bank and the second plurality of flattenedtube segments of said second tube bank from at least a leading face ofsaid first tube bank to a trailing face of said second tube bank andspanning said gap; a method for adjusting a ratio of the primary heattransfer surface area collectively defined by the first and secondplurality of flattened tube segments to the secondary heat transfersurface area collectively defined by the plurality of folded fin strips,the method comprising the step of: increasing or decreasing a depth ofsaid gap.